Photographs of momentous events and eventful moments are not necessarily photos that change the world, and more than a few of the 100 or so images in this enjoyable book are merely familiar photo-journalistic icons--mementos of important turning points, such as Wall Street flooding with shocked investors on the first morning of the Great Depression in 1929, or the signing of the Treaty of Versailles in 1919. Only the greatest of them are more than that: catalysts for social change or some new perception of life. It's thus arguable that John Paul Filo's justly famous 1970 image of a girl screaming in horror above the dead body of a student war protestor shot by the National Guard at Kent State University in Ohio, or Huynh Cong ("Nick") Ut's unforgettable 1972 mirror-image, of a screaming Vietnamese child running naked from her village after a napalm attack, are galvanic photos that helped turn the tide of public sentiment and political action.

Indeed, so many of these photos are of war or disaster that it's refreshing to note the few that seemed to provoke or promote change with no violent overtones. For example, Bert Reisfeld's charming publicity shot of a smiling, pelvis-swiveling Elvis Presley, his arms raised in triumph, seems a kingly benediction for the rock 'n' roll era. And the strangely airless, claustrophobic image of astronaut Yuri Gagarin, the first man in space, announces the arrival, at last, of the future. These images, of course, lack the resonance of the most artful photojournalism: Lewis W. Hine's early photograph of child labor in the U.S.--a small girl overshadowed by the industrial monolith of a cotton mill, seen in vast, receding perspective; or Dorothea Lange's classic 1936 "Migrant Mother," truly a Madonna of the Dust Bowl era, her haggard yet strong features suggesting reserves of dignity amidst despair.

Editor Peter Stepan has arranged these endlessly fascinating photos in a conventional, chronological fashion, leading us through the 20th century, and providing each image a page of helpful annotation and historical perspective. The grainy, cinematic, frozen moments that have come to define America's tragic sensibility--the assassination of John F. Kennedy, captured as he is shot in a speeding motorcade, or the image of a jetliner about to sunder the World Trade Center on September 11, 2001--are noted as much for their impact on the visual arts as for their impact on society, and that nicely broadens the dialogue of this collection. Bookended, logically, by images of two natural disasters--the 1906 San Francisco earthquake, and Hurricane Katrina's devastation of New Orleans in 2005--this tribute to photography also reminds us how the world changes us, and how the camera, so often a harbinger of change, is the ultimate witness as well.

Matt Damsker is an author and critic, who has written about photography and the arts for the Los Angeles Times, Hartford Courant, Philadelphia Bulletin, Rolling Stone magazine and other publications. His book, "Rock Voices", was published in 1981 by St. Martin's Press. His essay in the book, "Marcus Doyle: Night Vision" was published this past November.